Food intake and growth depressions, correctable by added dietary lysine, occur when certain amino acids are included in diets low in lysine. The most effective amino acids are those which compete with lysine for its transport into brain in vitro. Brain lysine content in vivo is often decreased by dietary amino acids, especially arginine. Studies of relationships among diet, blood and brain content of these amino acids and food intake are being continued. Dietary GABA reduces food intake and growth; the effect is lessened by increasing the protein content of the diet. Increased dietary protein increases activity of hepatic GABA transaminase and reduces levels of GABA in plasma, liver and kidney. Relationships among GABA intake, food intake and GBA metabolism are being investigated. The relationships among protein intake, plasma ratio of Trp to neutral amino acids and brain serotonin (5HT) concentration; and among energy intake, plasma Tyr/Phe and brain norepinephrine (NE) and dopamine (DA) content are also being studied. In studies to date a consistent relationship has not been observed between plasma Trp/NAA and protein consumption, or between plasma Tyr/Phe and energy intake. Also, a consistent relationship has not been observed between either whole brain levels of 5HT and protein intake, or between brain levels of NE and DA and energy consumption. Usually a narrow range of either protein or energy intakes has been associated with a wide range of neurotransmitter levels or vice versa. Effort is continuing to determine whether a clear relationship can be demonstrated over a narrow range of intake with specific dietary proteins.